An internal combustion engine providing in the engine exhaust passage an NOx storing catalyst storing NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or rich is known. In this internal combustion engine, the NOx produced when burning fuel under a lean air-fuel ratio is stored in the NOx storing catalyst. On the other hand, when the NOx storage ability of the NOx storing catalyst becomes close to being saturated, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby NOx is released from the NOx storing catalyst and reduced.
However, fuel and lubrication oil contain sulfur, therefore the exhaust gas contains SOx. This SOx is stored together with NOx in the NOx storing catalyst. However, this SOx is not released from the NOx storing catalyst by just making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOx stored in the NOx storing catalyst gradually increases. As a result, the NOx amount which can be stored ends up gradually decreasing.
Therefore, an internal combustion engine providing an SOx absorbent in the engine exhaust passage upstream of the NOx storing catalyst for preventing SOx from being sent to the NOx storing catalyst is known (see Japanese Patent Publication (A) No. 2000-145436). In this internal combustion engine, the SOx included in the exhaust gas is absorbed in the SOx absorbent, therefore SOx can be prevented from flowing into the NOx storing catalyst. As a result, it is possible to prevent storage of SOx from causing a drop in the NOx storage ability.
However, when using this SOx absorbent, if the SOx absorption ability of the SOx absorbent ends up becoming saturated, the SOx ends up flowing into the NOx storing catalyst. However, with this SOx absorbent, if raising the temperature of the SOx absorbent and making the air-fuel ratio of the exhaust gas flowing into the SOx absorbent rich, the SOx absorbent can be made to release the absorbed SOx and therefore the SOx absorbent can be restored. However, if making the SOx absorbent release SOx in this way, the released SOx ends up being stored in the NOx storing catalyst. Therefore, this internal combustion engine is provided with a bypass passage for bypassing the NOx storing catalyst. When making the SOx absorbent release the SOx, the released SOx is made to be exhausted through the bypass passage into the atmosphere.
As explained above, with the above-mentioned SOx absorbent, it is possible to raise the temperature of the SOx absorbent and make the air-fuel ratio of the exhaust gas flowing into the SOx absorbent rich so as to make the NOx absorbent release the SOx. However, in this case, the SOx is released from the SOx absorbent only at bit at a time. Therefore, to make the SOx absorbent release all of the absorbed SOx, the air-fuel ratio has to be made rich for a long period of time. Therefore, there is the problem that a large amount of fuel or reducing agent becomes necessary. Further, the SOx released from the SOx absorbent is exhausted into the atmosphere, so this is also not preferable.
When using an SOx absorbent in this way, if an SOx release control is not performed, the SOx absorption ability ends up being limited. Therefore, when using an SOx absorbent, an SOx release control must be performed. However, so long as performing an SOx release control, that is, making the SOx absorbent release SOx, the above-mentioned problem arises.